So while being the default (or only) store is clearly valuable, it is not the only path to a dominant position. As a Steam user, I find myself groaning if a game is exclusive to Epic or Battle.net (or whatever it's called now), but don't mind if I have to buy a game from GOG or a dedicated site put up by the developer. So the actual complaint isn't that the game isn't on Steam, it's that I don't want to have to install and use some shitty game launcher. I don't want it on my computer. People felt like this about Steam initially too, and it certainly has its shitty parts still, but it actually adds sone value over a raw installer exe, unlike some other stores.
It's all about the user experience. Minimal steps between them and the product. Buy on Steam? Great, I have that installed and ready to go. Buy on your website? Fine, I can autofill the purchase form, get an exe/zip, and I'm set. Buy on Epic? I have to install their store... then make an account... then add payment info... then find and purchase the game (while being bombarded with ads and flashy shit in an unfamiliar store interface). I think I'll pass.